The field of the invention is magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d), and particularly, studies which extend over a field of view which is larger than the static field of view of the MRI system. One such study is magnetic resonance angiography of human vasculature using contrast agents.
Magnetic resonance angiography (MRA) uses the nuclear magnetic resonance (NMR) phenomenon to produce images of the human vasculature. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins, and after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. Each measurement is referred to in the art as a xe2x80x9cviewxe2x80x9d and the number of views determines the resolution of the image. The resulting set of received NMR signals, or views, or k-space samples, are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. With conventional techniques, the total scan time is determined in part by the number of measurement cycles, or views, that are acquired for an image, and therefore, scan time can be reduced at the expense of image resolution by reducing the number of acquired views.
The most prevalent method for acquiring an NMR data set from which an image can be reconstructed is referred to as the xe2x80x9cFourier transformxe2x80x9d imaging technique or xe2x80x9cspin-warpxe2x80x9d technique. This technique is discussed in an article entitled xe2x80x9cSpin-Warp NMR Imaging and Applications to Human Whole-Body Imagingxe2x80x9d, by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, p. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then a signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (xcex94Gy) in the sequence of views that are acquired during the scan. In a three-dimensional implementation (3DFT) a third gradient (Gz) is applied before each signal readout to phase encode along the third axis. The magnitude of this second phase encoding gradient pulse Gz is also stepped through values during the scan. These 2DFT and 3DFT methods sample k-space in a rectilinear pattern.
To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. As described in U.S. Pat. No. 5,417,213 the trick with this contrast enhanced (CE) MRA method is to acquire the central k-space views at the moment the bolus of contrast agent is flowing through the vasculature of interest. Collection of the central lines of k-space during peak arterial enhancement is key to the success of a CEMRA exam. If the central lines of k-space are acquired prior to the arrival of contrast, severe image artifacts can limit the diagnostic information in the image. Alternatively, arterial images acquired after the passage of the peak arterial contrast are sometimes obscured by the enhancement of veins. In many anatomic regions, such as the carotid or renal arteries, the separation between arterial and venous enhancement can be as short as 6 seconds.
As indicated above, the acquisition of MRA data is timed such that the central region of k-space is acquired as the bolus of contrast agent arrives in the arteries of interest. The ability to time the arrival of contrast varies considerably and it is helpful in many applications to acquire a series of MRA images in what is referred to as a dynamic study which depicts the separate enhancement of arteries and veins. The temporal series of images from such a dynamic study is also useful for observing delayed vessel filling patterns caused by disease. This requirement has been partially addressed by acquiring a series of time resolved images using a 3D xe2x80x9cFourierxe2x80x9d acquisition as described by Korosec F., Frayne R, Grist T., Mistretta C., xe2x80x9cTime-Resolved Contrast-Enhanced 3D MR Angiographyxe2x80x9d, Magn. Reson. Med. 1996; 36:345-351 and in U.S. Pat. No. 5,713,358. However, with this method, the increased sampling rate of the center of k-space reduces the spatial resolution of the individual images in the time resolved series to about 75% of the resolution obtained when a single timed image is acquired during the passage of contrast.
There has been recent work using projection reconstruction methods for acquiring MRA data. Projection reconstruction methods have been known since the inception of magnetic resonance imaging. Rather than sampling k-space in a rectilinear scan pattern as is done in Fourier imaging and shown in FIG. 2, projection reconstruction methods sample k-space with a series of views that sample radial lines extending outward from the center of k-space as shown in FIG. 3. The number of views needed to sample k-space determines the length of the scan and if an insufficient number of views are acquired, streak artifacts are produced in the reconstructed image.
Efforts have been made to acquire CEMRA images in shorter scan times using undersampled projection reconstruction scanning methods. A method for reducing the number of projections in a 3D acquisition by a factor of two has been reported by F. Boada, J. Christensen, J. Gillen, and K. Thulborn, xe2x80x9cThree-Dimensional Projection Imaging With Half The Number Of Projectionsxe2x80x9d, MRM 37:470-477 (1997). Other methods are described in co-pending U.S. patent application Ser. No. 09/767,757 filed on Jan. 23, 2001 and entitled xe2x80x9cMagnetic Resonance Angiography Using Undersampled 3D Projection Imagingxe2x80x9d.
The non-invasiveness of MRA makes it a valuable screening tool for cardiovascular diseases. Screening typically requires imaging vessels in a large volume. This is particularly true for diseases in the runoff vessels of the lower extremity. The field of view (FOV) in MR imaging is limited by the volume of the B0 field homogeneity and the receiver coil size (typically, the FOV less than 48 cm on current commercial MR scanners). The anatomic region of interest in the lower extremity, for example, is about 100 cm and this requires several scanner FOVs, or stations, for a complete study. This requires that the patient be repositioned inside the bore of the magnet, the patient be re-landmarked, scout images be acquired and a preparation scan be performed for each FOV. All of these additional steps take time and, therefore, are expensive. When contrast enhanced MRA is performed, the repositioning also necessitates additional contrast injections.
Recently gadolinium-enhanced bolus chase techniques have been reported which overcome this difficulty, K. Y. Ho, T. Leiner, M. H. de Hann, J. M. A. van Engleshoven, xe2x80x9cGadolinium optimized tracking technique: a new MRA technique for imaging the peripheral vascular tree from aorta to the foot using one bolus of gadolinium (abs).xe2x80x9d Proc. 5th Meeting of ISMRM, p203, 1997. As described in U.S. Pat. Nos. 5,924,987 and 5,928,148, MRA data is acquired from a large field of view by automatically moving the patient table to a plurality of different locations during the scan and acquiring an image at each station. The movement of the table may be timed to follow the contrast bolus through the vasculature so that peak contrast is achieved at each station.
Prior moving table methods attempt to keep up with the contrast bolus as it moves through the vasculature by quickly moving to each successive station, stopping to acquire an image, and then moving to the next station. The resulting plurality of acquired images are then stitched together to form a single image. Typically, however, this stop-and-go imaging does not keep up with the moving bolus and images acquired near the end of the examination are not acquired under optimal contrast enhancement conditions.
While the present invention has particular clinical application to CEMRA, other studies in which image data is acquired over an extended field of view are known and present similar problems. As disclosed by Barkhousen, et al in xe2x80x9cWhole Body MR Imaging In 30 Seconds With Real Time True Fisp and A Continuously Rolling Table Platform: Feasibility Study,xe2x80x9d Radiology 2001:220:252-256 a human subject may be scanned over an extended field of view to locate tumors by moving the table.
The present invention is an improved MRI or MRA method which employs a projection reconstruction acquisition to continuously acquire k-space data as a subject is moved through the field of view (FOV) of an MRI system. Subregions in the subject along the direction of motion and the associated k-space data acquired while each subregion is within the MRI system FOV is phase corrected to a reference position, and an image is reconstructed for each subregion using the associated phase corrected k-space data.
A general object of the invention is to acquire a high resolution image with a MRI system by continuously moving the subject through the static field of view of the MRI system. The continuously acquired data is divided into sections and the views in each section are phase corrected to offset the effects of table motion. Subregions in the subject are depicted in the images reconstructed from the phase corrected sections of acquired data.
An object of the invention is to produce a CEMRA image which is acquired under optimal conditions as a contrast bolus moves through a subject""s vasculature. By continuously moving the patient table at substantially the same speed as the moving contrast bolus, k-space data is acquired at optimal contrast throughout the examination. No time is lost stopping and restarting table motion and higher resolution data may be acquired by performing a projection reconstruction acquisition.
Yet another object of the invention is to obtain higher time and spatial resolution images while tracking a contrast bolus through a subject""s vasculature. By using a projection reconstruction method for acquiring image data, a higher time resolution can be achieved without significantly sacrificing image quality. This enables reconstruction of CEMRA images with optimal contrast enhancement in each subregion.
Another object of the invention is to produce an image of prescribed FOV which is substantially larger than the MRI system FOV. The prescribed FOV in the subject is moved through the MRI system FOV and the resulting subregion images are concatenated to form a single image that spans the prescribed FOV.
Yet another aspect of the invention is to provide a temporal series of images of each anatomic subregion to study the dynamics of contrast flow within the subregion. This is achieved by acquiring the k-space data for each anatomic subregion as interleaved subsets of projection views and reconstructing an image from each interleave.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims herein for interpreting the scope of the invention.